1. Field of the Invention
The present invention relates to the deposition of material onto a semiconductor substrate using sputtering apparatus having a mechanical filter or screen for controlling the directionality of the material as it is deposited, such as a collimator plate or collimator tube. More particularly, the invention relates to reducing contamination caused by material which deposits on the filter or screen and then dislodges.
2. Background of the Prior Art
Sputtering is one well known method of depositing a film layer on a semiconductor substrate. A typical sputtering apparatus includes a target and a substrate support pedestal enclosed in a vacuum chamber. The target is typically affixed to the top of the chamber, but is electrically isolated from the chamber walls. A voltage source maintains the target at a negative voltage with respect to the walls of the chamber, thereby exciting a gas, which is maintained in the chamber at a low pressure, into a plasma. Ions from this plasma sputter the target.
As a first order approximation, the trajectories of the particles sputtered from any point on the target have a cosine angular distribution; that is, the density of sputtered particles ejected from a point on the target along a trajectory having a given angle from perpendicular to the target is proportional to the cosine of such angle. The target particles sputtered from the target generally travel in a straight line path and will tend to deposit on any surface that they contact.
One application of sputtering is to provide a conformal metal deposition layer on the surfaces of holes and trenches extending through one or more metal, dielectric or semiconducting film layers on the uppermost surface of the substrate. The metal deposited on the substrate by sputtering the target must form a continuous, i.e., conformal, coating on the wall and base of the holes or trenches.
The uniformity of the film layer deposited on the wall and base of each hole or trench is dependent on the angular distribution of the trajectories of the individual particles of target material reaching each of the holes or trenches. Particles travelling in paths that are substantially perpendicular to the substrate surface will pass through the open end of the hole or trench and deposit on the hole or trench base. Particles travelling at angles from perpendicular to the substrate surface will typically deposit on the hole wall, at the intersection of the hole wall and base, and at the upper surface of the substrate adjacent to the hole opening.
One means of providing uniform symmetric coverage of the holes requires collimation of the target material flux, such as with a target particle screening or blocking device. For example, as disclosed in U.S. Pat. No. 4,824,544, Mikalesen, et al., one method of collimating the flux is to locate a screening device such as a plate collimator having a plurality of relatively small holes therethrough between the substrate and the target. The holes have a ratio of length to diameter sufficient to screen target particles travelling in paths which are substantially oblique to the upper surface of the substrate from the flux of target particles passing through the plate collimator. The screening is provided because the obliquely travelling particles deposit on the wall of the collimator holes. Because the portion of the flux travelling obliquely to the substrate surface is screened out, the flux reaching the substrate has a distribution of trajectory angles close to perpendicular to the substrate.
One problem commonly associated with plate collimators is the formation of loosely attached deposits on the underside of the collimator and on the lower reaches of the collimator apertures. These deposits form when a target particle collides with the collimator surface at low kinetic energy. On the underside of the physical collimation device, the deposits are primarily created when target particles collide with another particle within the chamber but continue moving in an altered, low velocity, trajectory, which brings them in contact with the underside of the collimator. In the lower reaches of the collimator apertures, these deposits form when target particles collide with the aperture wall at a low angle trajectory. At the target end of the apertures, a large flux of particles traveling at large angles with respect to the wall surface are present and these particles impact the wall at sufficient energy to cover, or incorporate, any loosely attached deposits which form at the upper end of the aperture wall. However, at the end of the aperture wall located adjacent to the substrate, the surface of the collimator adjacent to the target blocks the target particles travelling at large angles with respect to the aperture wall. Therefore, the deposit at this location is primarily formed from particles travelling substantially obliquely to the aperture wall, which, when deposited, form loosely attached particles on the collimator surface.